6A.1 Development and Breakdown of Reverse Monsoon Troughs in the Western Pacific

Kelly Lombardo* State University of New York at Albany Albany, New York

1. INTRODUCTION patterns associated with each distinct feature. In the lower troposphere, the location of mean cyclonic vorticity The monsoon trough is a persistent lower highlights the axis of the monsoon trough. tropospheric feature in the Western Pacific during the Northern Hemisphere summer. It provides a potential 3. RESULTS source of low level vorticity for tropical cyclogenesis. Hence, the location and evolution of this feature becomes Figure 1 shows the mean winds and cyclonic important for the prediction of tropical cyclogenesis. vorticity at the 850 hPa level. Locations of initial tropical Lander (1996) identified the fundamental depression development were obtained from the Joint orientations of the monsoon trough, one of which is known Typhoon Warning Center (JTWC) best track data and as the reverse oriented monsoon trough. From this, plotted as well. Two reverse oriented monsoon troughs Lander made a comparison between tropical cyclone are visible in the August mean relative vorticity field. tracks under the typical monsoon trough and the reverse During this period, all instances of cyclogenesis occurred oriented (SW-NE) monsoon trough. Typically, tropical north of 20N and, more interestingly, most tropical cyclones moved northwestward due to the presence of cyclones formed within these two large-scale features. the monsoon trough. When the axis of the monsoon Approximately one month later, the monsoon trough trough became reversed, tropical cyclones tended to track returned to its typical climatological position, as shown in more northward. Furthermore, Harr and Elsberry (1995) figure 2. Again, the regions of cyclogenesis coincide with have identified six primary large-scale wind patterns in the the orientation of the monsoon trough. western Pacific utilizing EOF cluster analysis. Several of The simultaneous altering of the monsoon these patterns resemble the monsoon trough positioned trough’s axis along with the primary region of cyclogenesis along its various orientations. Once again, locations of occurs on two other occasions in this year alone. It is tropical cyclogenesis as well as tropical cyclone tracks evident that a relationship exists between these two appear to be influenced by these large-scale patterns. phenomena. Considering the above, it seems reasonable that Understanding this coexistence requires the understanding the mechanisms controlling the evolution examination of upper-tropospheric synoptic-scale of the monsoon trough can be crucial in providing some features. Mean relative vorticity and wind fields at 200 hPa insight to tropical cyclone formation and motion. Toward have been generated for the same two periods illustrated this end, this study will examine the influence of upper- in figures 1 and 2 (not shown). level synoptic-scale forcing mechanisms on the evolution Between 27 July and 26 August, upper-level of the monsoon trough. mid-latitude troughs repeatedly penetrated into the subtropics. As a result, the interaction between these two 2. DATA AND METHODOLOGY regions was tremendously enhanced. This was not the case during the second period. An elongated region of ECMWF gridded analysis have been utilized to high pressure, an extension of the upper-tropospheric examine the occasional reversal in orientation of the Asian anticyclone, was present over the mid-latitude monsoon trough. A twenty-day low-pass filter has been Pacific Ocean. In addition, the frequency of mid-latitude applied to the relative vorticity fields for several different troughs entering the subtropics decreased. Therefore, the summers, in order to highlight the strength and orientation magnitude of such interactions diminished considerably. of this feature. From the years examined, the 1988 period One can speculate that the penetration of mid- illustrates a clear reversal of the monsoon trough as well latitude troughs into the tropics may aid in the demise of as its dramatic impact on tropical cyclogenesis. Therefore, the monsoon trough, oriented along the climatological this year has been chosen for further investigation. position, and the development of this feature farther to the During the summer of 1988, four periods were north. The absence of such disturbances allows the selected isolating instances of both the monsoon trough typical monsoon trough to reestablish itself. Further and the reverse oriented monsoon trough. Time-averaged investigation is necessary to better understand these wind and relative vorticity were calculated for each period complex interactions and relationships. at two levels, 850 hPa and 200 hPa, to illustrate both the lower-tropospheric and upper-tropospheric large-scale ______* Corresponding author address: Kelly Lombardo, Department of Earth and Atmospheric Sciences, State University of New York at Albany, Albany, NY 12222. e-mail: [email protected] 4. ACKNOWLEDGEMENTS 5. REFERENCES

This work was supported by Office of Naval Harr, P. A. and, R. L. Elsberry, 1995: Large-scale Research Grant N00014-98-0599 and NSF Grant circulation variability over the tropical 9900671, under the advisement of Dr. John Molinari. western north pacific. Part I: Spatial patterns Programs used to filter data, calculate mean fields, and and tropical cyclone characteristics. Mon. plot tropical disturbances were provided by Dave Wea. Rev., 123, 1225-11246. Vollaro. Lander, M. A., 1996: Specific tropical cyclone track types and unusual tropical cyclone motions associated with a reverse-oriented monsoon trough in the western north pacific. Wea. Forec, 11, 170-186. (1)

Figure 1: Time-averaged wind and relative vorticity (shaded when cyclonic) for 26 July - 26 August 1988. Locations of initial tropical depression development during each given period have been plotted.

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Figure 2: Same as figure 1, but for 26 September - 6 November 1988.